Process for the production of stove-finished structural components from ageing-sensitive steel

ABSTRACT

The invention relates to processes for the production of a buckling-resistant stove-finished structural member from cold rolled and dressed strip (cold strip) non-ageing steel with high bake-hardening potential, more particularly of more than 70 N/mm&lt;2&gt;. The characterising feature of the invention is that the cold strip is converted by dressing into a yield point stretch-free state (Reh-Rel&lt;2 N/MM&lt;2&gt;), then stored at a temperature below room temperature and further processed into the form of a structural member, whereafter the strip is finally stove finished.

The invention relates to a process for the production of abuckling-resistant stove-finished structural member from cold rolled anddressed strip (cold strip) from non-ageing steel with highbake-hardening potential, more particularly of more than 70 N/mm².

To achieve a high bake-hardening potential as a rule use is made ofsteels which contain dissolved nitrogen as well as dissolved carbon.Examples of these are unkilled steels. The storage of strips of suchsteels at room temperature leads even after the short time of one or twodays to ageing processes which make impossible any satisfactory furtherprocessing, more particularly cold working. There is also an adverseeffect on the surface texture of the cold strips.

Ageing can be caused by the diffusion of dissolved carbon and/ornitrogen. In the case of pure carbon ageing the effect of temperature onageing time can be estimated as follows: The times t₁ and t₂ requiredfor identical ageing effects stand in converse ratio to the associatedtemperature-dependent coefficients of diffusion of carbon in α iron.

In the temperature range up to 100° C. we therefore have $\begin{matrix}\begin{matrix}{{t_{1}\quad {\left( T_{1} \right)/t_{2}}\quad \left( T_{2} \right)} = \quad {D\quad {\left( T_{2} \right)/D}\quad \left( T_{1} \right)}} \\{= \quad \frac{\exp \quad \left( {{- 21.1}/\left( {1,{987 \cdot 10^{- 3} \cdot T_{2}}} \right)} \right.}{\exp \quad \left( {{- 21.1}/\left( {1,{987 \cdot 10^{- 3} \cdot T_{1}}} \right)} \right.}}\end{matrix} & \lbrack 1\rbrack\end{matrix}$

with T_(1,2) in K.

Table 1 shows the factors calculated according to equation [1] for thedelay in time of an ageing effect due to lowered temperatures. Forexample, in comparison with ageing at room temperature, ageing at −10°C. lengthens ageing time by 62 times.

TABLE 1 Ageing temperature (° C.) 10 5 0 −5 −10 Factor¹⁾ 3.6 7 14 29 62¹⁾Factor for the delay in time of an ageing effect at differenttemperatures in comparison with room temperature for ageing by dissolvedcarbon according to equation [1]

The quantity of description of the effect of dissolved nitrogen on steelageing can be carried out similarly to the description of carbon ageingaccording to equation [1], using the coefficient of diffusion fornitrogen. The connection between ageing time and ageing temperature istherefore obtained as follows: $\begin{matrix}\begin{matrix}{{t_{1}\quad {\left( T_{1} \right)/t_{2}}\quad \left( T_{2} \right)} = \quad {D\quad {\left( T_{2} \right)/D}\quad \left( T_{1} \right)}} \\{= \quad \frac{\exp \quad \left( {{- 18.33}/\left( {1,{987 \cdot 10^{- 3} \cdot T_{2}}} \right)} \right.}{\exp \quad \left( {{- 18.33}/\left( {1,{987 \cdot 10^{- 3} \cdot T_{1}}} \right)} \right.}}\end{matrix} & \lbrack 2\rbrack\end{matrix}$

with T_(1,2) in K.

Table 2 shows the factors calculated according to [2] for the delay inageing effect caused by dissolved nitrogen.

TABLE 2 Ageing temperature (° C.) 10 5 0 −5 −10 Factor²⁾ 3.1 5.5 14 1936.5 ²⁾Factor for the delay in time of an ageing effect at differenttemperatures in comparison with room temperature for ageing by dissolvednitrogen according to equation [2]

It is an object of the invention to provide a process for theageing-free further processing of cold strips of an ageing-sensitivesteel with high bake-hardening potential to produce a stove-finishedstructural component.

To resolve this problem the invention provides a process as set forth inclaim 1 or a process as set forth in claim 3.

In the process according to claim 1 the ageing of dressed cold strip issuppressed by its storage at low temperature. In the alternative processset forth in claim 3, due to the bake-hardening effect triggered therebythe stove-finishing performed shortly after further shaping processingprevents the ageing of the cold strip dressed shortly prior to furtherprocessing.

To make use of the positive effect of a lowering of the surroundingtemperature during the storage of cold strips, the storage temperature Tin K (degrees Kelvin) can be estimated as follows, in dependence on theplanned storage time in hours:

 T=9225/(31.48−1n (48/t)  [3]

Equation [3] follows from equation [2] and relates to a steel which canno longer be satisfactorily processed, due to nitrogen ageing afterexceeding a storage time of more than 2 days at 20° C. In the case ofageing by both elements, it is enough to allow for nitrogen only, due tothe lower diffusion speed of carbon in comparison with nitrogen.

As an example, the change in material properties due to ageing atdifferent temperatures was measured on a cold strip of a steelcontaining 0.003% C, 0.27% Mn, 0.003% Si, 0.007% P, 0.006% S, 0.046% A1,0.001% N and Cu+Ni+Cr<0.1% (values in % by weight). After hot and coldrolling the steel was galvanised in a continuous fire-coatinginstallation with a maximum annealing temperature of 820° C. and thensubjected to 1.5% dressing. The difference between the upper and loweryield points (R_(eh)−R_(el)) was evaluated from the tensile test as ameasure of the risk of stretcher strains.

FIG. 1 shows the development in time of R_(eh)−R_(el) at roomtemperature, 60° C. and 100° C. Value R_(eh)−R_(el)=2 N/mm² can beregarded as the limit value for fault-free processing. With highervalues than 2 N/mm² the occurrence of stretcher strains must beexpected, since there is a marked drop in load in the stress/straincurve.

In FIG. 2 the associated time for reaching the value R_(eh)−R_(el)=2N/mm² is plotted Arrhenius-fashion for each temperature. As in the caseof all diffusion-controlled processes, the result in good approximationis a straight line.

The effect of a further lowering in temperature can be determined bylengthening the straight line with the values from Table 3.

TABLE 3 Ageing 30 20 5 0 −5 −10 temperature [° C.] Ageing 56 174 11182170 4320 8830 time³⁾ [h] (2, 3 (7, 3 (6, 7 (13 wks) (26 wks) (53 wks)days) days) wks)

While the critical value of ageing resistance is reached at 30° C. and20° C. after 2 and 7 days respectively, processing free from stretchstrains is ensured up to 13 weeks at 0° C. and even go up to one year at−10° C.

Table 4 lists the mechanical values of the steel, its 0.2% proof stress(Rp_(0.2)), tensile strength (Rm), elongation (A80), elongation withoutnecking (Ag), the r value and its bake-hardening potential BH₀, and alsothe contents of dissolved C and N in the starting condition.

TABLE 4 Rp_(0.2) Rm A80 Ag r BH₀ C_(diss.) N_(diss.) N/mm² N/mm² % %value N/mm² ppm ppm 215 310 44 23.5 1.75 73 30 <1

By the use of one of the two processes according to the invention asteel which has high contents of dissolved carbon and/or nitrogen and isnotageing-resistant at room temperature can be further processed withoutthe risk of surface faults even after a prolonged storage period.

The advantage of the process according to the invention lies in theutilisation of a high bake-hardening potential to produce steels fromwhich structural components can be made which have higher bucklingresistance in comparison with conventional bake-hardening steelsresistant to ageing at room temperature.

What is claimed is:
 1. A process for the production of a bucklingresistant stove-finished structural component from a cold strip whichcomprises ageing-sensitive steel with a high bake-hardening potential,comprising the steps of: converting the cold strip by temper rolling toa yield point elongation-free state in which the conditionR_(eh)−R_(el)<2 N/mm² is met, storing the cold strip at storagetemperature below room temperature for a storage-period whose length isat most equal to the length of the period at whose end the value ofcritical ageing is reached which results in dependence on the particularstorage temperature, cold working the cold strip to give a structuralcomponent, and stove-finishing the structural component, wherein saidbake-hardening potential is at least 70 N/mm².
 2. A process according toclaim 1, wherein the storage temperature T in [K] of the cold strip isselected in dependence on the planned storage time t in [h] inaccordance with the equation T=9225/(31.48−1n (48t)) with T: storagetemperature in [K] T: storage time in [h].
 3. A process for theproduction of a buckling-resistant stove-finished structural componentfrom a cold strip which comprises ageing-sensitive steel with abake-hardening potential, comprising the steps of: storing the coldstrip undressed for a storage period at room temperature, converting thecold strip by temper rolling to a state in which the conditionR_(eh)−R_(el)<2 N/mm² is met, clod working the temper rolled cold stripto give a structural component, and stove-finishing the structuralcomponents wherein said bake-hardening potential is at least 70 N/mm².